The invention is a media position alignment system for determining the position of the media relative to desired positions thereof in a media transporter in which media having opposed faces is transported along the path of a track including a spatially varying pattern on the media, the alignment system including a light sensor having a light-sensitive surface facing one face of the media in at least approximate registration with the track, the light-sensitive surface including plural sections, the sensor producing plural signals at an output thereof proportional to the intensity of light impinging on respective ones of the plural sections of the light-sensitive surface, a light source facing the other face of the media and directing a beam of light through the media onto the light-sensitive surface, whereby the plural signals vary as a function of the spatially varying pattern of the track as the media is transported along the path of the track, and signal detection and logic apparatus for converting the plural signals of the light sensor to a pair of coordinate positions used by control logic of the media transporter to position or align the media.
Electrostatic, electrophotographic, thermal and pen printers and plotters of the type which record an image on a strip of media moving longitudinally across a transverse linear print head are well-known. Various techniques are known for sensing media for different purposes. For example, U.S. Pat. No. 4,730,931 to Watson discloses a system which monitors light transmitted through the media to monitor fiber orientation in the media, but not for the purpose of positioning the media. U.S. Pat. No. 4,760,271 to Brenholdt discloses a quadrant sensor for monitoring roughness of webbed media. U.S. Pat. No. 3,886,542 to Ohtsuka discloses measuring displacement of a moving member by optically reading a scale with multiple paths and varying gradations imprinted on the member. U.S. Pat. No. 4,549,084 to Markel discloses a mask aligner used in semiconductor integrated circuit fabrication in which alignment errors between a wafer and a mask are sensed using reflected light. The present invention is directed to an improvement in media positioning systems in printers and plotters of the type discussed above. Such media positioning systems are of the type disclosed in U.S. Pat. No. 3,919,560 to Nopper and in U.S. Pat. No. 4,485,982 to St. John et al., in which a sensor senses light reflected from a pre-printed or coded track along one edge of a longitudinally transported strip of webbed media. In Nopper, track markings are reflectively sensed while in St. John et al. a coded track is reflectively sensed.
The media on which such apparatus records an image must be generally aligned with the media sensor. Some media transporters (such as, for example, some color printers) superimpose images on the media. Thus, each frame of the media must make several passes in order to record the several images. For this purpose, after the first image is recorded, the direction of the media travel is temporarily reversed until the media returns to the starting position of the second image. Then, the image is recorded on the media as the media is again transported in the forward direction. As can be appreciated, the accuracy with which the media is positioned prior to and during the recording of the subsequent images directly affects image quality. If there is an error in this alignment, the image is misaligned in proportion to the error. Such misalignment reduces the quality of the recorded image. As one example, in multi-image color recording, any misalignment may cause false colors to be recorded. The alignment of the media at all points during media transport is critical and directly affects image quality.
This problem is typically addressed in the prior art in the manner shown in FIG. 1 by the use of a pattern 10 recorded near the edge of the media 12 and a linear multi-pixel image sensor 14 (such as a charge coupled device (CCD)) connected to scan logic 16 which scans reflected light from the pattern 10 as the media 12 moves across the sensor 14 and develops a signal used by control logic of the printer or plotter to position the media 12. Typically, the recorded pattern 10 is a broken line with a series of rectangles along it. The CCD sensor 14 scans the media 12 and visually "reads" the pattern 10 to determine media position as the media 12 is returned to the starting position for the recording of the next image. The control logic of the printer or plotter processes the signal generated by the CCD sensor 14 to generate an error signal which is used to correct any misalignment of the media with respect to desired positions. The manner in which the signal generated by the CCD sensor 14 is used to correct the alignment of the media 12 with respect to the desired positions is well-known.
One significant problem with this prior art technique is that it is expensive to implement, in that it requires a linear array scanner or sensor 14 (typically a CCD) including all the timing and processing circuitry associated therewith. Another problem is that reflective mode sensing does not provide adequate signal-to-noise ratio with transparent media. Moreover, this technique has been found to produce error prone plots of marginal quality and requires a user alignment procedure.
Accordingly, it is an object of the present invention to accurately correct media alignment errors in a multi-pass media transporter without requiring a linear array scanner in order to reduce cost.
It is a further object of the invention to sense the two-dimensional position of the media from a track on the media using a quadrant photodiode sensor instead of a linear array scanner.
It is another object of the invention to sense the track with significantly improved signal-to-noise ratio even with transparent media.
It is a yet further object of the invention to sense the two-dimensional position of the media from the track without having to sense reflected light from the media but rather by sensing light transmitted through the media.
These and other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawings which accompany it.